1. Field of the Invention
The present invention relates to an array antenna calibration apparatus for use in a radio base station and the like.
2. Description of the Related Art
In order that a digital beam-forming apparatus forms an accurate transmission beam, it is necessary to make the phase characteristics and amplitude characteristics of signals emitted from respective antenna elements uniform.
FIG. 5 is a block diagram of a conventional array antenna calibration apparatus.
The array antenna calibration apparatus according to the conventional art includes beam formers 13 for users 1 to N, respectively, a user signal multiplexing section 12, multipliers 10, adders 5, transmitters 3, couplers 17, antennas 1, a power synthesizer 18, a receiver 7, a calibration factor calculation section 9 and a calibration signal generator 4.
Each beam former 13 forms a beam having a directivity for each user. The user signal multiplexing section 12 multiplexes the beams for the respective users 1 to N and outputs user multiplex signals for six transmitting systems, respectively. Each multiplier 10 multiplies the user multiplexed signal by a corresponding calibration factor. The calibration signal generator 4 generates a calibration signal corresponding to each user multiplexed signal. Each adder 5 adds the corresponding calibration signal to the corresponding user multiplexed signal multiplied by the calibration factor. Each transmitter 3 transmits the corresponding user multiplexed signal which is multiplied by the corresponding calibration factor and to which the corresponding calibration signal is added. The coupler 17 branches a part of each transmission signal and supplies the branched signal to the power synthesizer 18 and the remaining signal to the antenna 1. Each antenna 1 transmits the signal supplied from the coupler 17.
The power synthesizer 18 synthesizes the powers of the signals supplied from the six couplers 17. The receiver 7 receives the power-synthesized signals. The calibration factor calculation section 9 calculates a calibration factor for each user multiplexed signal based on the signal received by the receiver 7, and supplies the calculated calibration factor to the corresponding multiplier 10.
The calibration signals have such signal patterns to be orthogonal to one another among the transmitting systems. Due to this, the calibration factor calculation section 9 performs a correlation processing for the signals synthesized and received by the power synthesizer 18, whereby the phases and amplitudes of the calibration signals for the respective antenna elements can be measured. The calibration factor calculation section 9 also calculates the calibration factors of the respective transmitting systems based on the measured phases and amplitudes.
The above-stated conventional antenna array calibration apparatus has a disadvantage in that fluctuations in the characteristics of the couplers 17 and the antenna elements 1xe2x80x941 to 1-6 cannot be corrected. Further, although the conventional array antenna calibration apparatus can measure the characteristics of the couplers 17 and the antenna elements 1xe2x80x941 to 1-6 in advance and correct the fluctuations using a table, the apparatus disadvantageously requires high accuracy in measurement and stability in characteristics. In addition, to suppress a fluctuation in the characteristics of cables which connect the couplers 17 to the antenna elements 1xe2x80x941 to 1-6, it is necessary to arrange the couplers 17 in the vicinity of the corresponding antenna elements 1xe2x80x941 to 1-6. To do so, each coupler 17 needs a waterproof structure, with the result that the coupler becomes disadvantageously expensive.
To solve these disadvantages, a method adapted for an apparatus constituted as shown in FIG. 6 has been conventionally proposed. Namely, a calibration signal-receiving station 19 which includes a receiver 7 and a calibration factor calculation section 9 is disposed within a sight range. The receiver 7 receives calibration signals transmitted from base station array antennas 1xe2x80x941 to 1-6 and having signal patterns orthogonal to one another. The calibration factor calculation section 9 calculates calibration factors by measuring the phases and amplitudes of the respective signals. With this configuration, however, it is necessary to notify the obtained calibration factor to the correction factor receiving section 20 of each base station by a cable or radio communication means. As a result, the system becomes disadvantageously complicated and expensive. Further, it is disadvantageously necessary to dispose the calibration signal receiving station 19 within the sight range of the base station. Besides, it is disadvantageously necessary to grasp the accurate positional relationships between the base stations and the signal generating station.
The present invention has been achieved to solve the above-stated disadvantages. It is an object of the present invention to provide an array antenna calibration apparatus which is simple in configuration and inexpensive while ensuring an accurate calibration of an array antenna, and an array antenna calibration method therefor.
According to the present invention, there is provided an array antenna calibration apparatus comprising: supply means for supplying original calibration signals to a plurality of antenna elements constituting an array antenna, the original calibration signals being orthogonal to one another among the antenna elements; phase and amplitude characteristic calculation means for calculating correlations between calibration signals, emitted from the antenna elements and received by the adjacent antenna elements, and the original calibration signals related to the received calibration signals; relative calibration factor calculation means for obtaining a relative calibration factor among all the antenna elements constituting the array antenna based on phase and amplitude characteristics of the respective antenna elements; and calibration means for calibrating transmission signals to be supplied to the respective antenna elements based on the relative calibration factor.
In the array antenna calibration apparatus according to the present invention, the antenna elements constituting the array antenna may be classified into a first group and a second group, and the relative calibration factor calculation means comprises: first relative calibration factor calculation means for obtaining a relative calibration factor among all of the antenna elements belonging to the first group based on the phase and amplitude characteristics of all the antenna elements of the first group; second relative calibration factor calculation means for obtaining a relative calibration factor among all of the antenna elements belonging to the second group based on the phase and amplitude characteristics of all the antenna elements of the second group; third relative calibration factor calculation means for obtaining a relative calibration factor between the first group and the second group based on the phase and amplitude characteristics of one of the antenna elements belonging to the first group and the phase and amplitude characteristics of one of the antenna elements belonging to the second group; and fourth relative calibration factor calculation means for obtaining a relative calibration factor among all the antenna elements constituting the array antenna based on the relative calibration factor among all the antenna elements belonging to the first group, the relative calibration factor among all the antenna elements belonging to the second group, and the relative calibration factor between the first group and the second group.
The array antenna calibration apparatus according to the present invention may comprise: synthesizing means for synthesizing the calibration signal received by one of the antenna elements belonging to the second group from one of the antenna elements belonging to the first group with the calibration signal received by the one antenna element belonging to the first group from the one antenna element belonging to the second group, and the relative calibration factor between the first group and the second group may be obtained based on the phase and amplitude characteristic obtained by the phase and amplitude characteristic calculation means based on the synthesized calibration signal.